WASHINGTON - Youngsters who use marijuana are more likely to develop serious mental health problems, the government said Tuesday. A private group said law enforcement increasingly is targeting people who smoke and deal the drug.

Past medical studies have linked marijuana with a greater incidence of mental disorders such as depression or schizophrenia. But questions remain about whether people who smoke marijuana at a young age are already predisposed to mental disorders, or whether the drug caused those disorders.

Government officials say recent research makes a stronger case that smoking marijuana is itself a causal agent in psychiatric symptoms, particularly schizophrenia.

"A growing body of evidence now demonstrates that smoking marijuana can increase the risk of serious mental health problems," said John P. Walters, director of the White House Office of Drug Control Policy.

Administration officials pointed to a handful of studies to make their case. One, from the Substance Abuse and Mental Health Services Administration, found adult marijuana smokers who first began using the drug before age 12 were twice as likely to have suffered a serious mental illness in the past year as those who began smoking after 18.

The ratio was 21 percent to 10.5 percent. Those who first started as teens also were at significantly higher risk.

Also Tuesday, The Sentencing Project released a report that found the government's "war on drugs" has become the "war on drug" as police agencies increasingly target marijuana.

Begun in the 1980s, the war on drugs was aimed at stopping large-scale narcotics traffickers, particularly those selling cocaine. But since 1990 more of the focus has been on catching users and low-level dealers. And more often than ever, the drug targeted is marijuana, according to the group, a national nonprofit organization that works on judicial reform and favors alternatives to jail.

Of some 700,000 marijuana arrests in 2002, 88 percent were for possession, it said. And only one of every 18 of those arrests ended in a felony conviction.

"Arresting record numbers of low-level marijuana offenders represents a poor investment in public safety" and diverts resources from "more serious crime problems," said Ryan King, co-author of the report.

King found that in 1992 arrests for heroin and cocaine comprised 55 percent of all drug arrests and marijuana 28 percent. A decade later heroin and cocaine arrests accounted for less than 30 percent of all arrests, while marijuana's share had risen to 45 percent.

Jennifer deVallance, spokeswoman for the White House drug office, said there are many reasons for the greater focus on marijuana. Among them: Marijuana is the single largest drug of abuse in the nation, the strains are more potent than ever and more is known about health dangers.

"For the first time, more kids are seeking treatment for marijuana use than alcohol," she said.

The Sentencing Project called for renewed national discussion of the war on drugs, an idea echoed by the conservative American Enterprise Institute. The group reported last month that despite spending at about $40 billion a year now and toughening drug sentencing laws, "America continues to experience the Western world's worst drug problems."

An epidemic of heroin use more than three decades ago, followed by a 1980s epidemic of cocaine and crack, prompted a massive intensification in drug enforcement while giving short shrift to prevention and treatment, the institute reported. It decried budgeting that spends two-thirds of drug control funds on enforcement, 25 percent on treatment and just 12 percent on prevention.